The capability for extra-vehicular activity (EVA) contributes significantly to the overall productivity of humans in space. A dramatic increase in the number and complexity of tasks assigned to the pressure-suited astronaut characterizes the historical evolution of EVA from Gemini through Apollo. The Skylab EVAs demonstrated that the fundamental creativity and flexibility of humans in the space environment could compensate for mechanical and structural failure. The EVAs performed during Skylab 2 were relatively simple in terms of support equipment, yet they were crucial to the success of the mission. The Skylab Airlock Module played a critical in these EVAs and provided the first precedent for a space station EVA airlock. The first data on airlock sizing equipment stowage and free volume was obtained during the Skylab mission, as described in Skylab Experience Bulletin No. 2, Architectural Requirements for Airlock, Maynard Dalton Johnson Space Center, NASA TM 85539, June 1974.
Since 1982, the Space Transportation System (STS) places increasing emphasis upon the regular use of EVA for in flight development, recovery and repair of space systems. This trend is expected to continue on the Space Station. The dexterity of the human operator in EVA is transmitted through the human/machine interface imposed by the protective envelope of the suit; hence, significant advances in EVA capability are due primarily to improvements in the design of the space suit. The primary goal of space suit design is to reduce losses in human dexterity and in mobility to the extent possible within safety constraints. The shuttle Extravehicular Mobility Unit (EMU) represents a refinement in technology over the cumbersome pressure suits of the first EVAs. Despite these improvements, the use of EVAs on space stations will require a new generation of space suits with capabilities that exceed those of the current EMUs. In addition, the demands and constrains imposed by planned space station EVA operations necessitate the development of new hardware and procedures for EVA support, including servicing of the suit and its life support systems, suit donning and doffing and an efficient system for egress from and ingress to the space station. Further details on EVA state of the art are provided in Cohen and Bussolari, Human Factors in Space Station Architecture II, EVA Access Facility: A Comparative Analysis of Four Concepts for On-Orbit Space Suit Servicing, NASA TM 86856, April 1987.
Beginning in the 1960s NASA Ames Research Center has conducted several studies aimed at developing the technology of EVA suits. The Ames AX series suits demonstrated the use of low-friction rotary bearings between segments to achieve exceptional mobility in a hard suit. The current AX-5 suit under development at Ames Research Center incorporates state-of-the-art technology to provide excellent mobility and low leakage in a low maintenance suit with a long projected surface life. This totally hard suit is durable enough to allow work in areas that may have sharp edges and pointed structures. It is equipped with replaceable sizing rings to permit major suit parts to be used by crew members of differing body sizes. The modular construction of the suit will enable rapid repair or service change out of individual suit parts. Such suits are described, for example, in the following issued U.S. Pat. Nos. 3,405,406, issued Oct. 15, 1968 to Vykukal; 3,636 564, issued Jan. 25, 1972 to Vykukal; 4,091,464, issued May 30, 1978 to Vykukal; 4,151,612, issued May 1, 1979 to Vykukal; 4,593,415, issued June 10, 1986 to Vykukal; 4,594,734, issued June 7, 1986 to Vykukal; and 4,598,428, issued July 8, 1986 to Vykukal. U.S. Pat. No. 3,583,322, issued June 8, 1971 to Vykukal relates to a locomotion and restraint aid for EVA.
Conventional EVA as practiced on the STS utilizes "prebreathing." Prebreathing is a procedure by which the crewmember breathes pure oxygen for a number of hours prior to beginning EVA. Prebreathing reduces the risk of decompression sickness (the "bends"): the development of nitrogen bubbles in soft tissue and body fluids that may occur if suit operating pressure is low with respect to the space station cabin pressure. When the initial cabin pressure for an STS based EVA is 14.7 p.s.i., the prebreathing time is approximately 3 hours. In order to reduce this prebreathing time penalty, an operational protocol has been developed to gradually lower the cabin pressure to 10.2 p.s.i. prior to beginning prebreathing. The required prebreathing time under this protocol is reduced to approximately 1 hour.
Although this prebreathing protocol appears to be appropriate for STS operations, it would not be appropriate for the Space Station. The Space Station internal environment will be much more complex than that of the Space Shuttle. With a greater emphasis on scientific and commercial users, the requirement to accommodate these payloads in an atmosphere that regularly fluctuates between 10.2 and 14.7 p.s.i. would impose a severe hardship on users. In particular, it would become necessary to develop pressure variation testing programs on Earth for each of the the scientific and commercial users. Such a requirement would not only add cost and complexity, but for life sciences experiments especially would introduce additional variables. Finally, there would be an added cost associated with testing for flammability, toxicity and outgassing of all materials at the lower mean pressure, but with a higher partial pressure of oxygen. The arguments for a 14.7 p.s.i. space station atmosphere are persuasive. The AX-5 suit design enables the elimination of prebreathing, resulting in a first order time savings of from 1 to 3.5 hours for an EVA, depending on the air pressure used in the space station.
The current STS orbiter airlock is not pumped down for EVA. The atmosphere is bled off to vacuum and and sacrificed. This procedure is satisfactory for the shuttle, with a maximum of three EVAs per 10 day flight. However, for a space station, with daily EVAs, partly closed life support system and sensitivity to external contamination, sacrificing atmosphere is not acceptable. Therefore, it will be necessary either to pump down and save the atmosphere or to find ways to minimize or eliminate pump down.
It is also known in the art to provide access to a suit through an attachment connecting the back of the suit to a wall. Such an arrangement for a radiation protection suit is disclosed in Takimoto, Japanese Published Patent Application (Kokai) No. 54-8297, published Jan. 22, 1979. In that publication, the suit remains open to atmospheric pressure and remains attached to the wall while it is in use. An access port in the back of a pressure suit through which an astronaut enters the suit, but which remains open to atmospheric pressure, used in a weightlessness simulation system and process, is disclosed in U.S. Pat. No. 4,678,438, issued July 7, 1987 to Vykukal.
While a substantial amount of work has been done on egress from and ingress to a space vessel for EVA in space, a need remains for further improvement of egress and ingress techniques in order to meet the demands of a space station, in which egress and ingress will be a much more routine procedure than in the past.
Although the problem of prebreathing has been removed by the innovation of the AX-5 one atmosphere suit, the possibility exists that a crew member might get the bends or an embolism if he or she chose to operate the suit at a pressure lower than 14.7 p.s.i. for the purpose of obtaining greater dexterity and flexibility. Therefore, it is a requirement of the space station program that at least one of the two baseload airlocks have a "hyperbaric" capability. In hyperbaric mode, the afflicted crew member would be placed in the hyperbaric chamber with a paramedic trained companion in attendance and both would be pressurized quickly to to six atmospheres (90 p.s.i.) in order to force nitrogen bubbles in the blood back into solution. The pressure is then slowly reduced to 14.7 p.s.i. The problem of incorporating hyperbaric function into an airlock is complicated by the fact that electrical equipment cannot be allowed to operate within the chamber, so the pump must be outside the airlock, but controlled from inside. Also, the structural weight penalties of holding 90 p.s.i. can be significant in a non-optimum pressure vessel geometry.